Method of reducing multipath effects



`April 7',v 1942.

c. w. HANSELL METHOD OF REDUCVING MULTIPATH EFFECTS Filed' March 15, 1940 o/scRM/NAMR Fan Der No. 2

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ATTRNY.

Patented Apr. 7, 1942 f y METHOD oF REDUomG MULTIPATHvr v EFFECTS Clarence W. Hansell, Port Jefferson, N. Y., asi signor to Radio Corporation of vcorporation of Delaware America, a

Application March 15, 194i), Serial No. 324,052

claims. (c1. 25o-e) This invention is directed toward reducing the effects of multipath transmission in phase or frequency modulation communicating systems, and particularly toward improvement of radio communication over long distances. The invention is especially applicable to systems employing 'frequencies in the range of about 1.5 to 30 megacycles (wavelengths between and 200 meters) and which require transmission via .the ionosphere and the surface of the earth.

It has been known in the radio art that long distance short wave radio transmission is aecompanied by arrival of signal power at the receiver over more than one path. The multiple paths are produced because the waves which reach the receiver have been refracted and reected between the ionosphere and the earth a different number of times, because of refraction from different ionized layers or regions in the ionosphere and because of magneto ionic splitting of the waves into components of different velocities. There have also been many instances of transmission simultaneously around the world in different directions.

A common result of the multiple path transmission between transmitter and receiver is that useful signal modulations arriving at the receiver are badly distorted and do not faithfully reproduce the modulations of the transmitter.

When it is attempted to employ phase or fre- I quency modulation on long distance short wave circuits, the receiver must deal with multiple received currents of random phase relations which bring in modulations arriving at different times. As a consequence, during frequency modulation, the components of current received simultaneously may be different in frequency and this causes distorting phase modulation beats between the currents. At the same time, very low modulating frequency energy components carried by the several paths tend to add in like phase to produce a useful output while higher frequency components of modulation addsuccessively less effectively and produce proportionally less useful output.

The principles of the present invention are based on an appreciation of the fact that if identical modulations are sent out over a'number of transmitters of closely adjacent carrier frequencies, the distortions in their modulations at a receiver, will not be alike, and the phase modulation beats between received current components of different frequency will have random phase relations in the received currents for the several transmitters. Consequently, by transmitting the same modulation onseveral transmitters on closely adjacent frequencies, and combining their several received currents we may obtain a reduction in the relative amount of distortion of the modulations. Although it is entirely possible and practical to improve long distance short wave radio transmission by employing and operating several transmitters, and their corresponding receivers, on closely adjacent frequencies, all carrying the same modulations simultaneously, with the modulationsl combined for utilization in the output circuits of the receivers, such a system would be relatively costly to construct and maintain. In accordance with the present invention, I propose to obtain an equivalent result by means of a single transmitterand receiver. y

The following is a more detailed description of the invention, accompanied by a drawing wherein Figs. l, 2 and 3 are graphical representations given for purposes of exposition in order to aid in an understanding of the invention; and Figs. 4 and 5 illustrate, by way of example only, transmitter and receiver circuits, respectively, which can be used to carry out the principles of the invention. 1

For purposes of illustrating the principles of the invention, let us assume that we wish to transmit phase or frequency modulated radio telephone signals across the North American Continent on some radio carrier frequency between 1.5 and 30 megacycles. It has been found by tests that when this is attempted the received signals are badly distorted due to multiple radio paths extending from the transmitter to the receiver. The telephone modulations, for reasonably good quality service, when multipath effects lare absent, may be assumed to require modulating frequencies in the band from 30 to 6000 cycles Der second.

In accordance with the present invention, I apply to the transmitter, in addition to the signal modulation, an additional constant modulation at some higher frequency, say 7000 cycles per second. The amount of 7000 cycle modulation is variable according to circumstances but a peak phase deviation of 1.25 radians is suggested. This corresponds to a peak frequency deviation of plus and minus 8750 cycles per second. Asa result of this '1000 cycle modulation the radio frequency carrier is reduced in power and power is added at a number of uniformly spaced side frequencies (since the total output power is constant). The vequivalent continuous wave frequency spectrum produced by the 1.25 radians peak phase deviation at '7000 cycles per second, and the approximate relative amplitudes of each frequency component of current is illustrated diagrammatically in Fig. 1. As shown in the figure, the result of the 7000 cycle modulation is to divide up the power of the transmitter into a number of frequency components, equivalent to a number of transmitters operating on carrier frequencies spaced at intervals of 7000 cycles. The percentage amplitude of each of the frequency components of Fig. 1 is appropriately indicated, and it will be appreciated that the power at all amplitudes of the frequency components add up to 100%, which is the same as the total power of the carrier if it were not modulated.

If now, in addition to the 7000 cycle continuous sinusoidal modulation, We apply to the transmitter in accordance with the invention, a phase or frequency modulation produced by telephone currents then each of the original component side frequencies will be phase or frequency modulated in like degree by the useful signal. The result is nearly the same as if we transmitted the useful modulation by means of a number of separate transmitters on closely adjacent carrier frequencies, since each original component side frequency can be considered as a carrier.

At the receiver it is proposed in accordance with the invention that the whole significant band of transmitted frequencies be amplified selectively, heterodyned to a fixed intermediate band of frequencies separated by frequency selection from interference, amplitude limited and applied to back-to-back frequency modulation detectors (sometimes called off-tuned detectors) having discriminator circuits broad enough to include all of the significant transmitted frequencies. Fig. 2 illustrates the approximate fre- From Fig. 3 it will be apparent that the amplitude and phase errors of component radio frequencies at the receiver may be quite serious in producing distortions of the modulation transmitted over a single carrier but that the distortions may balance out to a large extent when signals are sent on a plurality of carriers as illustrated in Fig. 1. In practice it is found that currents are received over long distance short wave radio circuits by way of a considerable number of paths and that the received currents have quency response of the receiver circuits, including the back-to-back or balanced discriminator circuits of the balanced frequency modulation detectors.

With a phase or frequency modulation system as described, the useful modulation carried by the several equivalent radio carriers tends to -f add in producing useful signal output from the receiver. However, multipath distortions of the useful modulations do not tend to add nearly so well but, instead, tend to balance out to a large extent.

By plotting curves showing typical steady state multiple circuit amplitude and phase or frequency change responses over the whole significant frequency band, it will be found that the errors in amplitude and phase in the portions of the band about each of the several equivalent carriers are not alike but some are in one direction and some in another so that, when the errors are combined in their effect, they tend to average out, thus effecting the balancing out of distortion.

' Fig. 3 illustrates the effect of the invention in a simple case, and shows the amplitude and phase errors in steady state response which would result if we had one meter wavelength main radio path paralleled by another path of greater length such that a time delay of 0.2 millisecond resulted, when the current received over the second path has a strength about 58% of that received over the first path. For this condition the second path would be about 60,000 meters, l

or 2000 wavelengths longer than the rst path. Our amplitude and phase errors would then pass through a whole cycle of variation for a frequency change of one part in 2000, or every 5000 cycles.

different time delays and different strengths. Furthermore it is observed that the paths are continually changing. As a consequence, conditions are usually far more complex than in the simple case illustrated in Fig. 3, although it should be understood thatl the invention functions to reduce the resultant distortions in these complex situations.

Fig. 4 shows one form of transmitter circuit which can be employed in connection with the receiver of Fig. 5 to obtain the results of the present invention. The circuits of Figs. 4 and 5 are typical of any suitable transmitting and receiving apparatus which can be used and are not to be construed as limiting the invention to any particular circuit.

More specifically, Fig. 4 is a frequency modulator circuit employing a microphone circuit or other source of a signal wave 2, whichfmay be coupled to a suitable amplifier 4, and from which the signal energy is conveyed over transformer I4 and a wire line 6 to suitable carrier modulating apparatus B. Element I0 is a source of constant frequency modulation whose frequency is higher than the range of signal frequencies produced by the microphone circuit v2.v The constant frequency energy from oscillator ID is sent over transformer I2 and out over line 6 to apparatus 8. If preferred, the element I0 and its coupling into the transmitter circuits may be located at the transmitting station.

Apparatus 8 comprises any suitable vacuum tube apparatus for generating a suitable high frequency constant amplitude carrier wave whose frequency will vary at the rate of the modulation frequency of the signal wave and of the frequency of the oscillator IIJ. Apparatus 8 is herein shown in box form since it may take any desired known form such as that illustrated and described in Dome Patent No. 1,917,102, July 4, 1933, or in copending Crosby application Serial No. 136,578, led April 13,1937, Figs. 1 and 2, or copending Crosby application Serial No. 311,074, filed December 27, 1939. See also my United States Patents Nos. 1,787,979, 1,803,504, 1,819,508, 1,830,- 166, and 2,027,975.

The frequency modulated output of the apparatus 8 is coupled to a power amplifier I6 and/or limiter and/or frequency multiplier, from which the output energy may be conveyed over high frequency lines or radiated from a suitable antenna I8.

As an illustration of suitable frequencies which may be employed in the operation of the system, the speech modulating frequencies may require a band of from 30 to 6000 cycles per second, the oscillator l0 a constant frequency of 7000 cycles per second, while apparatus 8 may generate a carrier in the range between 1.5 to 30 megacycles.

The system of the invention, it will be understood, is applicable to both 4phase and frequency modulation systems.

A suitable receiver for the transmitter of Fig. 4 is shown in Fig. 5 in Which the carrier energy is picked up in a suitable antenna circuit 20 and selectively amplified at 22 before being supplied to a heterodyne combining detector where it beats with energy from a local heterodyne oscillator 26 to produce a fixed band of intermediate frequencies. The intermediate frequency output from detector 24 is then passed through selective filters and the amplitude thereof limited in ap-paratus 28 before being passed on to back-to-back frequency modulation detectors 30 having discriminator circuits broad enough to include all of the significant transmitted frequencies. The back-to-back detectors (sometimes called oiT-tuned detectors) translate the incoming frequency modulated energy to amplitude modulated energy having a substantially true reproduction of the original frequency modulation.

Alternative receiver arrangements are described in my United States Patents Nos. 1,813,922, 1,867,567, 1,922,290 and 1,938,657. The approximate resonance characteristics of the two detector tube circuits in 30 is clearly illustrated in Fig. 2 which shows that the curves have maxima for frequencies which lie on either side of the mean or carrier frequency. An arrangement of. back-to-back detector circuits which is known in the art and can be used is described in Usselman Patent No. 1,794,932, granted March 3, 1931, to whichreference is herein made.

The output from the detector circuit 30 passes through a low pass filter 32 which passes the useful modulation and suppresses the 7000 cycle continuous modulation of oscillator l0, after which the energy can be translated in a headphone or loudspeaker or passed on over a line to a central station.

Further improvement in the reduction of multipath distortion may be obtained. by employing highly directive antennae (preferably low angle) and space diversity receivers in connection with my invention, such receivers using spaced antenna systems.

It is also realized that the use of my invention may reduce the signal to noise ratio in the received signals but this reduction can be tolerated for the sake of reducing distortion due to the multipath phenomenon, since it is the distortion rather than noise which causes the greatest difficulty in providing telephone service on long distance high frequency circuits.

The invention also finds use in the ultra short wavelength field below ten meters where multipath effects are encountered in line of sight transmission, due to reflections from buildings in those cases where the modulation frequencies are sufficiently high so that the difference in time of travel over the circuits approaches or exceeds the time of a half cycle of the modulation. Obviously, although I have described the invention as applied to radio telephone systems, it is equally applicable insubcarrier multiplex telegraph and in facsimile communications systems and to any other systems requiring similar characteristics. In the application of the invention, the transmitter modulation current may I itself be a current which is amplitude, phase or frequency modulated by a signal current. In other words, the modulation current may be a subcarrier current which is itself modulated by signaling currents. Also, there may be a number of subcarrier currents, each modulated by the same or different signal modulations.

What is claimed is:

l. The method of reducing multipath distortions which includes continuously modulating the Wavelength of a carrier with a single frequency unmodulated current whose frequency is outside the range of signal modulating frequencies, and simultaneously therewith also modulating said carrier with said signal modulating frequencies, transmitting the resultant modulaa current of constant frequency which is higher than the speech modulating frequencies, transmitting the resultant modulations, receiving the band of transmitted frequencies, heterodyning the received energy to obtain frequency modulated energy of intermediate frequency, demodulating the intermediate frequency energy, and discarding the modulation of constant higher frequency completely for all purposes.

3. The method of reducing multipath distortion in a radio communication system which includes frequency modulating a carrier in the range between 1.5 and 30 megacyclesboth with speech waves having frequencies in the band from substantially 30 to 6000 cycles per second and also with a constant audio frequency which is higher than 6000 cycles, transmitting the resultant modulations, receiving the band of transmitted frequencies, heterodyning the received energy to obtain frequency modulated energy of intermediate frequency, demodulating the intermediate frequency energy, and translating only the demodulated energy having the speech modulations, whereby the demodulated constant audio frequency component is discarded for all purposes,

4. The method of reducing multipath distortion in a communication system which includes modulating the wavelength of a high frequency carrier with signal energy covering a band of low frequencies and also with energy of constant frequency higher than said band, transmitting and receiving the resultant modulated carrier energy, demodulating the received energy by producing amplitude modulations of the carrier energy which are substantially proportional yto the original frequency modulations, and rectifying the carrier current, then translating only the rectified energy having the signal modulations, whereby the demodulated constant frequency component is discarded for all purposes.

5. The method of reducing the effects of multipath effects between a radio transmitter and a radio receiver which comprises modulating the wavelength of a transmitter carrier continuously at a frequency lying above the band of signal modulating frequencies, receiving the resultant transmitted modulations, and separating the signal frequency components from the higher frequency component and utilizing only said separated signal frequency component, whereb-y the received higher frequency component is discarded for all purposes.

CLARENCE W. HANSELL. 

